Soybean milk is employed not only as materials of Tofu (bean curd), but also as beverages that are directly taken by people as functional foods in recent years. The soybean milk is usually produced as follows. First, soybeans are washed with water for removal of dust and so on. Then, the soybeans are soaked in water at the normal temperature of volume three times as much as the soybeans, and are held in the soaked condition for 5 to 20 hours depending on the season. During such a period, imbibition and germination of the soybeans occurs. Then, the imbibed soybeans are put in a grinding device, such as a mixer and a mortar, and are ground at the normal temperature while water at the normal temperature is added in volume about 5 to 6 times as much as the soybeans. Raw soybean juice (or soybean juice) is thereby produced. After heating the raw soybean juice for about 3 to 15 minutes, it is subjected to a solid-liquid separation process at high temperatures using a solid-liquid separation device, such as a centrifugal separator and a filter press. As a result, soybean milk and bean curd lees are obtained.
The soybean milk thus produced is provided as Tofu after being added with a coagulant, e.g., bittern, gluconolactone and calcium sulfate, or as beverages after being directly packed in a container and then sealed off. However, beverages containing soybean milk have unpleasant tastes, such as green grass smell and a bitter taste, due to the presence of small amounts of ill-smelling components including 2-hexenal and several kinds of saponins, and those unpleasant tastes have hitherto invited the biggest difficulty in utilization of the soybean milk. Various proposals on lactic bacteria fermentation using lactic bacteria have been made for a taste improvement of the soybean milk. But those proposals for masking the unpleasant odor of a soy bean cancel a body taste and Umami (savory taste) specific to the soy bean. Therefore, the problem of the unpleasant odor and taste specific to the soy bean is not yet overcome at the present.
It is known that soybean protein has an effect of reducing cholesterol. That fact has increased the number of people eating Tofu recently. However, containers, sauces and spices are required when eating Tofu, and places where people can eat Tofu are limited. On the other hand, when beverages containing soybean milk are packed in containers and sealed off in a portable way, there are no limitations on places for people to drink the packed beverages and people can easily take in the soybean milk as functional foods anywhere.
Meanwhile, with recent widespread diffusion of preventive medicine knowledge, it has become a prevailing practice for an increasing number of people to preventively take natural origin foods, which have physiological activities, in a healthy or half-healthy condition before they suffer from diseases, for the purpose of keeping themselves healthy. Among those foods, fish such as a horse mackerei, bonito, mackerei and sardine contains a small amount of effective natural compounds, i.e., polyunsaturated fatty acids represented by eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), in the form of glycerides. Those natural compounds have lately received keen attention because useful physiological activities, such as a cerebral nerve activation action, a blood cholesterol and lipid lowering effect, and an antiallergic effect, have been found therein one after another in recent years. It has therefore been proposed to not only take those natural compounds as functional foods, but also to mix them in various foods.
EPA and DHA are typical ones of polyunsaturated fatty acids and are relatively abundantly contained in fat of fish (referred to as a “fish oil” hereinafter), such as fatty meat of a horse mackerei, salmon roe, yellow tail, mackerei, saury, eel, pilchard, rainbow trout, salmon, and saurel. EPA and DHA contained in the fish oil have physiological activity effects, such as a platelet aggregation inhibitory effect, a blood triglyceride lowering effect, and a blood VLDL and LDL cholesterol lowering effect, and hence have preventive and remedial effects for arteriosclerotic diseases. It is also known that depletion of DHA lowers the memory learning ability. While EPA and DHA have, on one side, have many physiological activity effects mentioned above, the fish oil containing EPA and DHA has, on the other side, a peculiar foul odor. Also, those polyunsaturated fatty acids are very easily oxidized because EPA has twenty carbons and five double bonds in a molecule and DHA has twenty-two carbons and six double bonds in a molecule. It is further known that the EPA and DHA generate an unpleasant odor and taste with deterioration of the specific taste.
Heretofore, various types of soybean milk products have been proposed in consideration of the state of the art described above. As one example of the prior art, Japanese Patent Laid-Open Publication No. 7-255406 discloses a method of producing soy bean processed food enriched with polyunsaturated fat, wherein soybean milk containing polyunsaturated fat is produced by mixing soybean milk and about 1% to about 25% of polyunsaturated fat with respect to the weight of soybean protein in the soybean milk. According to the detailed description in the Publication, “a maximum amount of EPA(DHA)-containing liquid fat uniformly dispersible in 18 liters of soybean milk (about 650 g of protein content) is 160 g (about 25% with respect to the weight of protein in the soybean milk)”, and “if the fat content exceeds about 25%, the fat is not uniformly dispersed and an oil component is separated”.
As another example of the prior art, Japanese Patent Laid-Open Publication No. 10-42819 discloses “a method of producing a beverage containing DHA-mixed soybean milk, the method comprising the steps of adding and mixing soybean milk and a DHA-containing fish oil at a weight ratio of 1:0.2 to 1, thereby preparing an emulsion of the DHA-containing fish oil, and further diluting the emulsion with the soybean milk”.
Further, Japanese Patent Laid-Open Publication No. 6-90662 discloses “a method of producing fermented milk wherein, when producing fermented milk containing DHA and EPA, DHA and EPA are mixed in raw milk which is then subjected to lactic bacterial fermentation, or DHA and EPA are mixed in fermented milk which has been subjected to lactic bacterial fermentation”.